The field of the invention is that of transmitting digital data, in particular by radio. More precisely, the present invention relates to a method enabling a service channel to be transmitted in frames that are plesiochronous relative to said service channel, and the invention also relates to a corresponding transmission system.
Two signals are said to be xe2x80x9cplesiochronousxe2x80x9d when the ratio of their frequencies includes an irrational fractional part. The description below is given in the context of a transmission system, e.g. by microwave beams, in which two types of digital data are conveyed between a transmitter and a receiver. The transmitter and the receiver may either be endpoints of a transmission system, or they may be repeaters installed between such endpoints.
The two types of digital data are as follows: customer data, also referred to as xe2x80x9cpayloadxe2x80x9d data, and service data. Customer data makes up a payload channel and is transmitted in a radio frame which is plesiochronous relative to the service channel. By way of example, the payload channel has a data rate of 40 Mbps (megabits per second) while the service channel has a data rate of 64 Kbps.
By way of example, the service channel satisfies CCITT Recommendation G703 concerning clock accuracy and pulse shaping. Conventionally, this service channel contains supervisory data. It can also contain voice data, e.g. from PCM encoding of speech data.
It is known to insert service channel data into reserved locations in the radio frames that serve, outside said reserved locations, to convey the payload data. At the receiver of these radio frames, service channel data is extracted after the frame has been synchronized and in the knowledge of the positions of said reserved locations. However, when the service channel data is made up of blocks of predetermined length (e.g. 8-bit bytes from PCM encoding), it is necessary to provide the receiver with a clock for such blocks (byte clock) in order to enable it to recover the blocks that have been transmitted. Under such circumstances, it is known to transmit the block clock on an auxiliary channel.
The drawback of that solution is that it requires the presence of the auxiliary channel, thereby reducing the performance of the system in terms of transmission efficiency.
A particular object of the present invention is to mitigate that drawback.
More precisely, an object of the invention is to enable service channel data blocks to be transmitted in radio frames that are plesiochronous relative to said service channel, and in which the block clock that enables the data blocks to be recovered is not transmitted over an auxiliary channel, i.e. in which the block clock can be recovered at the receiver merely by analyzing the frames it receives.
This object, together with others that appear below, is achieved by a method of inserting a service channel in frames that are plesiochronous relative to the service channel, the method consisting also in transmitting a block clock between a transmitter and a receiver of the frame, the method consisting at the transmitter in reserving at least one location in the frame for conveying the data of the service channel.
According to the invention, the method consists in:
at the transmitter:
i) inserting blocks of the service cannel into the reserved location, causing each of the blocks to be preceded by a predetermined number n1 of identical bits referred to as xe2x80x9cstartxe2x80x9d bits, and do so in compliance with the following rules:
in the event of there being no need to pad out the remainder of the location, repeating the method at step i for a new block;
in the event of it being necessary to pad out the remainder of the location, causing each of the blocks to be followed by a number n2 of identical bits referred to as xe2x80x9cstopxe2x80x9d bits, the stop bits being different from the start bits, and repeating the method from step i for a new block; and
at the receiver:
detecting the regular arrival of start bits or the regular arrival of start bits preceded by stop bits, so as to be able to decide that blocks are present in the reserved locations and to extract the blocks from the reserved locations.
The invention is particularly applicable to transmission by radio, and it also provides a transmission system in which the method is implemented.